Battery performance is known to be influenced by temperature, so how sensitive is the Pipistrel Velis Electro to cooler temperatures? If cold batteries lose capacity and take hours longer to charge, that will mean fewer flights in a day. What should pilots and operators of electric aircraft expect as the mercury dips? We highlight some of our findings from the first cool days below.
As autumn brought frosty mornings, we compared the performance of the e-plane to that of warmer temperatures. A series of ground runs were performed to compare discharge rates at 3°C and 5°C with those at 15°C and 20°C from earlier in the year. In each case, the motor was run at a steady cruise power setting, 20 kW, and the discharge rate measured in %SOC/min (% State of Charge per minute). The results are shown in Graph 1 where lowest discharge rate (best endurance) was from the ground runs at 15°C and 20°C outside air temperature (OAT) in May. The discharge rates at 3°C and 5°C were slightly higher, indicating that the energy in the battery would be used 5% to 10% faster. This difference could be caused in part by the lower temperature. However, the lower State of Health (SOH), which represents the total capacity of the battery, could also explain the difference. The SOH decreased from 97% in the spring discharges to 84% to 89% in the fall. It’s worth noting that SOH may also be temperature dependent, as we have noticed the SOH go up and down as temperatures fluctuate day-to-day. In each ground run, the rate of discharge rose slowly as the SOC decreased from 90% to 30%. As Graph 1 illustrates, the discharge rate during the spring when the battery was at 30% to 40% SOC was similar to the discharge rate in the fall with the battery at 80% to 90% SOC.
Gabriel charging the Velis Electro
Graph 1: Battery Discharge Rate
Given the small differences in discharge rates at the various temperatures, it is not surprising that a broad operating range (-20°C to 35°C) is given for the Velis in the Pilot Operating Handbook. In other words, the impact of cooler autumn temperatures is small on discharge rates and might reduce flight endurance by a couple of minutes.
Next, we looked at the charge rates to see how temperature impacts charging the battery.
Previous studies showed that the battery charger operated at its maximum charge rate when the battery temperature was above 19°C. The maximum rate is maintained until 91% SOC, at which point it tapers or slows down as it approaches 100%.
The frosty morning experience demonstrated a much slower charge rate when the outside air temperature was 3°C to 5°C and the battery temperature was 8°C to 10°C (Graph 2). Instead of the typical one hour, as shown in the columns on the left of the graph, it took six hours to charge the battery on the morning of 1 November!
Graph 2: Time to Charge the Velis Battery
The normal charging rate of 10kW per battery was reduced to a charge rate that started at just 1kW. A closer look at the charge rate revealed strong temperature sensitivity (Graph 3). The charge controller limited the charge rate to 1kW per battery when the battery temperature was below 12°C. At 12°C the charge rate increased to 2kW, and at 13°C it increased to 3kW with similar steps up to the maximum rate of 9kW to 10kW per battery when the temperature is above 20°C. This is a striking demonstration of the temperature sensitivity of charge rates.
Note: The graph also shows a few points where the charge rate is 3kW at higher temperatures. These points were caused by setting the maximum charge rate manually. There are five charge rate options that an operator can select on the charger. When choosing a manual setting below the maximum, the charger reports a slightly higher charge rate (0.1kW), so an operator may choose to use the lower setting and hope to speed up the process while the battery is cold. However, leaving the maximum set at a lower charge rate after the battery has warmed up to 15°C will actually extend the charging time, so being aware of this curve will help future charging decisions.
Graph 3: Charging Power vs. Cell Temperature (Battery 1)
Battery temperature is typically 5°C to 10°C higher than the outdoor temperature (Graph 4). For example, if the plane is in a heated hangar overnight, the thermal inertia causes the temperature to only change slowly when the plane is brought out into colder outside air. The result is a higher battery temperature than the surrounding air. An example is an initial battery temperature of 15°C when the air temperature is 7°C, the grey lines in Graph 4. Similarly, the charging process has current flowing through the battery, and this heats the battery during the charging process. The extent of the heating depends on the amperage or current flowing into the battery. In the case of the battery starting at 15°C, it gradually rose to 20°C and then charged at full speed despite an outside temperature of 7°C. However, when the outdoor temperature was less than 5°C and the battery temperature was 10°C, the slow charge was not sufficient to raise battery temperature above 10°C and it remained a slow charge.
Graph 4: Air Temperature vs. Battery Temperature for Charging
The observed reduction in battery charge rates would clearly impact available flying hours and flight training operations in Canada. The same challenge is found in European countries like Sweden, Norway and Switzerland. Pipistrel has addressed the issue with a battery pre-heater that uses the existing liquid battery cooling circuit to heat the battery to 20°C so that it can be charged at its normal rate.
So, what is the conclusion? How do frosty mornings and cooler temperatures impact e-plane operations?
First, the good news. Discharge rates or flight endurance are only slightly changed over a broad range of temperatures.
Second, the not-so-good operational news. Charging takes longer when the battery temperature is below 20°C, and much longer when it is below 12°C. The charge controller is set to lower charge rates to protect the battery from high amperage during cold temperatures. Protecting battery life is good, but long charge times reduce available flying hours, so operators want a solution and a battery pre-heater has been created and certified.
Waterloo has ordered the battery pre-heater for the Pipistrel Velis Electro and looks forward to measuring how effective this solution is to the frosty morning charging challenge. Stay tuned.